Technical Field
The present disclosure relates to the field of bandgap circuits. The present disclosure relates more particularly to a low temperature drift bandgap circuit in integrated circuit dies.
Description of the Related Art
Integrated circuits often include reference voltage generators that generate various reference voltages. The reference voltages can be used in a large number of applications including accurate reading of memory cells, phase locked loops, voltage controlled oscillators, analog circuits, digital signal processing circuits, etc. It is beneficial for a reference voltage to have a particular don't value without variation data processing or environmental factors.
Bandgap voltage generators are often used to generate a reference voltage that can be used in any circuit applications. Bandgap voltage generators rely on the bandgap between the conduction band and the valence band of a semiconductor. Bandgap energy is the energy required for an electron to make the transition from the valence band of a semiconductor material to the conduction band of the semiconductor material. Each semiconductor material has a bandgap particular to that material. Because the bandgap energy is a physical characteristic of the semiconductor material it can be relied on as a reference voltage to which other voltages can be compared. Thus, bandgap voltage generators that generate a voltage based on the bandgap of a semiconductor material are commonly used in integrated circuits in which a reliable reference voltage is desired.
In spite of the constancy of the bandgap energy, bandgap voltage generators are imperfect. Bandgap voltage generators include circuitry such as transistors, resistors, and amplifiers that imperfectly reproduce the bandgap voltage. In particular, bandgap voltage generators may generate a voltage that varies unacceptably with changes in temperature. This is due to problems that can occur and processing of the integrated circuit die.
FIG. 1 is a schematic diagram of a known bandgap voltage generator 20 implemented in integrated circuit die with a monocrystalline silicon substrate. The bandgap voltage generator 20 generates a bandgap reference voltage VG based on the bandgap energy of monocrystalline silicon.
The bandgap voltage generator 20 includes a first group of p type bipolar transistors Q1. In the example FIG. 1, there are n transistors Q1 connected in parallel with each other. The emitters of the transistors Q1 are coupled to the non-inverting input of an operational amplifier 22. The collector and base terminals of the transistors Q1 are coupled to ground.
Bandgap voltage generator 20 further includes a second group of p type bipolar transistors Q2. An example of FIG. 1, there are n*m transistors Q2 each connected in parallel with each other. Thus, the number of transistors Q2 is the number of transistors Q1 multiplied by a number m. The emitters of the transistors Q2 are coupled to a resistor R1. The base and collector terminals of the transistors Q2 are connected to ground.
The resistor R1 is coupled between the inverting input of the amplifier 22 and a resistor R2. The resistor R2 is coupled between the inverting input of the amplifier 22 and the train terminal of a PMOS transistor M1. The gate of the transistor M1 is coupled to the output of the amplifier 22. The source of the transistor M1 is coupled to the supply voltage VDD.
A resistor R3 is coupled between the non-inverting input of the amplifier 22 and the train terminal of a PMOS transistor M2. The gate of the PMOS transistor is coupled to the output of the amplifier 22. The source of the PMOS transistor M2 is coupled to VDD.
The output of the bandgap voltage generator 20 is the node between the resistor R3 and the drain of the transistor M2. The output of the bandgap voltage generator generates the bandgap voltage VG based on the bandgap of the semiconductor substrate.
The reference voltage VG is based on the base emitter voltage Vbe1 of the transistors Q1 and the factor m. In particular, the voltage VG is given by the following relation:
                                                        VG              =                            ⁢                                                Vbe                  ⁢                                                                          ⁢                  1                                +                                  Δ                  ⁢                                                                          ⁢                  Vbe                  *                  R                  ⁢                                                                          ⁢                  2                  ⁢                                      /                                    ⁢                  R                  ⁢                                                                          ⁢                  1                                                                                                        =                            ⁢                                                Vbe                  ⁢                                                                          ⁢                  1                                +                                                      (                                          kb                      *                      T                      ⁢                                              /                                            ⁢                      q                                        )                                    *                                      (                                          ln                      ⁡                                              (                        m                        )                                                              )                                    *                  R                  ⁢                                                                          ⁢                                      2                    /                    R                                    ⁢                                                                          ⁢                  1                                                                                                                            (              1              )                                                                          (              2              )                                          where kb is Boltzmann's constant, T is the absolute temperature in kelvin, q is the charge of an electron. This can be written in simpler terms as:VG=VC+VP*K  (3)whereVC=Vbe1,  (4)VP=ln(m)*Kb*T/q  (5)andK=R2/R1  (6)The term VC is complementary to absolute temperature (decreases with increases in absolute temperature). The term VP is proportional to absolute temperature (increases with increases in absolute temperature). K is the ratio of R2 and R1.
Designers of a bandgap voltage generator 20 according to FIG. 1 typically try to design the circuit so that the temperature complementary term VC and the temperature proportional term VP balance each other over a wide range of temperatures so that the generated bandgap voltage VG varies little with temperature.
FIGS. 2A and 2B illustrate two graphs showing the dependence of Vbe1 and ΔVbe on temperature. In the example of FIG. 2A, Vbe, which corresponds to VC in equation 3, varies by −2 mV per degree rise in Celsius, whereas ΔVbe, which corresponds to VP in equation 3, varies by 0.08 mV per degree Celsius. It can be seen that these two values of VC and VP do not cancel each other out well. However, in the graph of FIG. 2B the term VP is multiplied by the constant K which is the ratio of R2 to R1. When multiplied by the factor K, VP more closely cancels VC as can be seen in the curve labeled VG, which is the sum of the two and is thus the final bandgap voltage VG from equation 3. The generated bandgap voltage VG curve on the graph on FIG. 2B has only a mild curve with changing temperature. The constant K is selected to minimize the change in the generated bandgap voltage VG with temperature.
However, this solution suffers from some drawbacks. In particular, the absolute value of the base emitter voltage varies with the processing carried out on the semiconductor substrate during manufacture. Room temperature Vbe may vary slightly from one die to another based on processing. Furthermore, the slope of Vbe will vary with processing so that the VP and VC do not cancel the same way on each die. Thus, the bandgap voltage may drift with temperature from die to die.
These drawbacks can be seen with respect to the graphs in FIG. 2C. The upper graph on FIG. 2C discloses several curves of Vbe for different processes carried out to make a die. The middle line, labeled VBE_BTYP, starts at about 730 mV at the low temperature of −40° and drops to about 420 mV at a high temperature of 120° C. The upper line, labeled VBE_BIMIN, starts at about 850 mV and decreases to about 480 mV. The lower line, labeled VBE_BIMAX starts at about 700 mV and decreases to about 400 mV with increasing temperature. This graph also shows that for different processes Vbe starts at different values at room temperature.
The lower graph of FIG. 2C shows three curves representing the slope of Vbe for different processes. As can be seen, the slopes of Vbe with respect to temperature (dV/dT) are different for the three different processes. Thus, a single design for a bandgap voltage generator will produce different bandgap voltages based on the process steps carried out in the manufacture of the semiconductor die.